use_backend is working perfectly,but I don’t know how to set the same expression for nbsrv:
nbsrv([%[req.hdr(host),lower,map(/etc/haproxy/backend.map,bk_default)]])

This is the error which haproxy returns when I try to restart:
error detected while parsing ACL ‘MAIN_not_enough_capacity’ : missing comma after conv keyword ‘map’ in ACL expression ‘nbsrv([%[req.hdr(host),lower,map(/etc/haproxy/backend.map,bk_default)]])’.